Chromatic dispersion on an optical fiber transmission line temporally fluctuates with the external temperature, stress or the like. In an ultra high speed optical transmission system, the power to withstand of an optical signal transmitted on the optical fiber transmission line against the chromatic dispersion lowers considerably. Therefore, the quality of the transmitted signal degrades even if there is a slight fluctuation in the chromatic dispersion. It is, therefore, necessary to appropriately the equalize dispersion arising due to the fluctuation in the chromatic dispersion on the optical fiber transmission line. One approach to compensate for the temporal fluctuation of chromatic dispersion arising is the automatic dispersion equalization technique. In order to construct an ultra high speed optical transmission system that uses the automatic dispersion equalization technique, it is essential to establish a control method that can be easily implemented.
As the automatic dispersion equalization technique, a control method based on the magnitude of the extracted clock component of a received optical signal is disclosed in Japanese Patent Application Laid-Open No. 11-68657 (Method and system for controlling chromatic dispersion and a method for detecting amount of dispersion), for example. Moreover, methods for controlling a variable dispersion equalizer based on the result of monitoring the errors of a transmitted optical signal are disclosed in Japanese Patent Application Laid-Open Nos. 9-326755 (Automatic equalizing system) and 2001-77756 (Optical fiber communication system incorporating automatic dispersion compensation module for compensating variation of dispersion caused by temperature).
However, the control method disclosed in Japanese Patent Application Laid-Open No. 11-68657 has a disadvantage in that control differs according to waveforms or degrees of chirping of signals to be used.
For example, if a chirp-free NRZ (Non-Return-to-Zero) optical signal is linearly transmitted, an optimum point is obtained when the magnitude of a clock component becomes a minimum. If a chirp-free RZ (Return-to-Zero) optical signal is linearly transmitted, an optimum point is obtained when the magnitude of the clock component becomes a maximum. In addition, if frequency chirp occurs to the signal waveform of, for example, an NRZ signal, then an optimum point is not necessarily obtained when the magnitude of the clock component becomes a minimum.
Thus, in controlling the variable dispersion equalizer by clock extraction, a control method need to be changed according to transmission conditions, and therefore, this approach is not a versatile approach. In addition, in the method for monitoring the degradation of signal quality due to different factors, e.g., the method for monitoring the magnitude of the extracted clock component, generally requires preparation of different monitors for the respective factors of the degradation of the signal quality. Therefore, the configuration of the apparatus becomes complicated.
In the method disclosed in Japanese Patent Application Laid-Open Nos. 9-326755 and 2001-77756, by contrast, optimum control can be ensured irrespective of factors for the degradation of signal quality and transmission conditions by controlling the variable dispersion equalizer so that transmission errors finally become a minimum.
If the controlling of the variable dispersion equalizer using a transmission error monitor is actually applied to an optical communication terminal station, the following problems should be taken into account. It is necessary that the receiving terminal can identify a transmitted optical signal in order to monitor transmission errors and control the variable dispersion equalizer based on an error state. In addition, a state in which errors can be accurately counted is essential. For instance, in an terminal station constituted to transmit an optical signal using an error-correction code, if no optical signal is input to an optical receiver, the out-of-frame synchronism for the structure of frames encoded by the error-correction code and error correction and error detection functions cannot be, therefore, fulfilled, or if the error number detection function of the error-correction code cannot be used because errors exceeding the error-correcting capability of the code occur, then the variable dispersion equalizer cannot be controlled by monitoring transmission errors. Therefore, to monitor transmission errors and control the variable dispersion equalizer, it is very important that signal quality can be always, accurately monitored.
However, the method for controlling the variable dispersion equalizer as well as the apparatus in a state in which transmission errors cannot be detected is not disclosed in the conventional art, for example, in Japanese Patent Application Laid-Open Nos. 9-326755 and 2001-77756). It is, therefore, necessary to further contrive to incorporate the controlling of the variable dispersion equalizer based on the transmission error monitoring result into the optical communication terminal station.
It is an object of the present invention to solve at least the problems in the conventional technology.